The present invention relates to an epoxy resin composition, to prepreg and to fibre-reinforced composite materials. More particularly, it relates to prepreg, fibre-reinforced composite materials and honeycomb sandwich panels, and to the epoxy resin composition employed as the matrix resin therein, favourably used in the production of structures where, as advanced composite materials, high levels of compression/tensile strength and tensile/flexural modulus are demanded, together with high levels of specific strength and specific modulus comprising these properties divided by specific gravity.
Fibre-reinforced composite materials have outstanding mechanical properties and are widely used in aircraft, in motor vehicles and in industrial applications but, as their applications have become more and more diverse, so have the property requirements placed on them.
Fibre-reinforced composite materials are heterogeneous materials in which the essential constituents are reinforcing fibre and matrix resin, and so there are considerable differences between the properties in the fibre axial direction and the properties in other directions. Thus, the resistance to a drop-weight impact is governed by the interlaminar delamination strength which is determined for example by the interlaminar edge delamination strength, so it is well known that merely increasing the strength of the reinforcing fibre does not lead to fundamental improvements. In particular, fibre-reinforced composite materials in which the matrix resin is a thermosetting resin reflect the low toughness of the matrix resin and tend to show ready failure in the case of stresses applied in other than the fibre axial direction. Hence, in addition to improving the properties in the fibre axial direction, various techniques have been proposed with the objective of improving the composite material properties in directions other than that of the fibre axis.
As a means for enhancing the toughness of the thermosetting resin itself, there is disclosed in U.S. Pat. No. 4,656,208 the addition of an aromatic thermoplastic resin oligomer to the epoxy resin, and it is said that impact strength of the fibre-reinforced composite material is also improved.
Furthermore, in U.S. Pat. No. 3,472,730 (1969), there is disclosed the improvement of the interlaminar delamination strength by providing a separate exterior film comprising an elastomer-modified thermosetting resin at one or both faces of a fibre-reinforced sheet.
However, not only is the improvement effect in terms of the interlaminar delamination strength and the like inadequate, these methods also each have their own disadvantages.
In the method of enhancing the resin toughness by the addition of an aromatic thermoplastic resin such as a polysulphone, an increase in the resin viscosity is unavoidable, so that either the impregnation of the fibre is inadequate or, alternatively, prepreg thoroughly impregnated with resin is inferior in its handling characteristics in that its drape is unsatisfactory. Furthermore, as the amount of thermoplastic resin is increased, the solvent resistance of the cured product is reduced.
Moreover, in the method in which there is interposed film containing elastomer-modified thermosetting resin, as the elastomer content is increased so the heat resistance of the composite material obtained falls considerably while, conversely, as the elastomer content is reduced so there is a marked deterioration in the interlaminar delamination strength improvement effect.
As an attempt to resolve such problems, prepregs have been proposed with resin fine particles dispersed at the surface. For example, in U.S. Pat. No. 5,028,478, there is disclosed a technique for providing a tough composite material of good heat resistance using fine particles of a thermoplastic resin such as nylon.
However, in the method disclosed in U.S. Pat. No. 5,028,478, because of the high dependence on the toughness of the thermoplastic resin itself, when the composite material is exposed to severe environmental conditions for a long time, and/or when there is poor affinity between the fine particles of thermoplastic resin and the bulk resin, interfacial separation between the bulk resin/fine particles is brought about and there is a danger of a considerable lowering in the toughness between layers.
In European Unexamined Patent Publication Nos 377,194 and 392,348, there are disclosed techniques for providing a composite material which is outstanding it its heat resistance and toughness by using thermoplastic resin fine particles of polyimide or polyethersulphone. The fine particles used dissolve in the bulk resin at the time of the prepreg curing, to form a thermoplastic resin layer, and the toughness of the composite material is enhanced by this. However, in these technologies, since there is employed a means in which thermoplastic resin fine particles dissolve in the bulk resin, the internal state of the finally-formed fibre-reinforced composite material, and in particular the interlaminar thickness between the layers of composite material comprising prepreg or the like, is markedly influenced by changes in the fabrication conditions such as the pressure and the rate of temperature rise, and so there is the disadvantage that the properties of the composite materials obtained are unstable.
Now, in the case of aircraft structural materials or interior materials, from the point of view of reducing weight there has been increasing use of honeycomb sandwich panels in which the skin panels are fibre-reinforced composite materials. Here, the honeycomb sandwich panels are generally produced by so-called co-cure fabrication, in which there is used an aramid honeycomb, glass honeycomb or aluminium honeycomb as the honeycomb core, and prepregs for forming the skin panels are laid on both faces thereof, after which the curing of the resin and adhesion to the honeycomb core are simultaneously effected.
In said co-cure fabrication, hitherto there has mostly been used a fabrication method in which an adhesive film is interposed between the honeycomb core and prepreg laminate but, recently, from the point of view of further reducing the weight of the honeycomb sandwich panel and lowering cost, there has been a demand for a so-called self-adhesive technology in which the honeycomb core and the prepreg are directly affixed. However, in cases where no adhesive film is used, the resin contained in the prepreg needs to bear the burden of adhesion to the honeycomb core, and so it has been difficult to ensure good adhesion.
As a honeycomb fabrication method relating to prepreg in which carbon fibre is the reinforcing fibre, and relating to the matrix resin, U.S. Pat. No. 4,500,660 discloses an epoxy resin composition which contains the reaction product of a specified epoxy resin and a butadiene-acrylonitrile copolymer with functional groups at both terminals, plus dicyandiamide as a curing agent, for the purposes of improving the peel strength to the honeycomb core and the interlaminar shear strength in terms of the skin panels. However, while it is possible by the technology described in U.S. Pat. No. 4,500,660 to maintain, to a certain degree, high levels of room temperature strength properties such as tensile strength in the composite material obtained, the peel strength between the prepreg and the honeycomb core is still inadequate and there is the disadvantage of poor wet heat resistance.
The present invention aims to offer fibre-reinforced composite materials which can be used favourably in applications where high level properties are demanded in a hot and wet environment in particular, and which are outstanding in their impact resistance and various characteristics of strength such as tensile strength, compression strength and interlaminar delamination strength; and also an epoxy resin composition and prepreg with outstanding handling properties which can be suitably employed in the production of such fibre-reinforced composite materials; and prepreg which is outstanding in its peel strength to a honeycomb core.
In order to overcome the aforesaid problems, the present invention has the following constitution. That is to say, it is an epoxy resin composition for a fibre-reinforced composite material, where the glass transition temperature Tg of the cured material obtained by heating for 2 hours at 180xc2x0 C. is at least 150xc2x0 C., and the modulus of rigidity Gxe2x80x2R in the rubbery plateau in the temperature region above the aforesaid Tg is no more than 10 MPa.
Furthermore, in order to resolve the aforesaid problems the present invention also has the following constitution. Specifically, it is a prepreg formed by the impregnation of reinforcing fibre with an epoxy resin composition which includes the following constituents [A], [B] and [C], plus curing agent, and the respective contents per 100 parts by weight of the total epoxy resin in said resin composition are 5 to 35 parts by weight of constituent [A] and 50 to 95 parts by weight of constituent [B], and at least 90% of the constituent [C] is contained within a depth, from the prepreg surface, of 20% of the average prepreg thickness.
[A] trifunctional epoxy resin and/or tetrafunctional epoxy resin
[B] difunctional epoxy resin
[C] fine particles of average particle size 3 to 70 xcexcm which are substantially insoluble in the epoxy resin of the aforesaid resin composition.